Derailleur variable transmission systems usually rely on the position of a drive chain, belt or the like relative to driving and driven gears or cogs that engage the drive chain. Usually, there are a number of driving gears of varying diameter and a number of driven gears of varying diameter. A desired gear ratio can be selected by displacing the drive chain with a suitable mechanism from one driving and/or driven gear to the other.
Bicycles predominantly use such transmission systems. In bicycles, the system can include two or more front driving gears or chainrings and usually up to eleven driven gears or sprockets. Thus, the highest gear ratio is achieved when the chain is engaged with the largest chainring and the smallest sprocket. The lowest gear ratio is achieved when the drive chain is engaged with the smallest chainring and the largest sprocket.
In competitive cycling or when there are a number of external factors that require attention, it can be difficult for a cyclist to ascertain what gear ratio is required for his or her preferred cadence. For example, the cyclist may have been cycling with a particular gear ratio for a significant duration and he or she may have forgotten the combination of chainring and sprocket that is being used. He or she could easily make an incorrect selection. For example, the next gear ratio down could require a change of chainring. Not realising this immediately could cause the cyclist loss of precious time. Also, it can be dangerous, particularly at high speed and/or in a peloton, if it becomes necessary for the cyclist to look down at the gears.
Many bicycles have front and rear derailleur transmission systems. Such bicycles are typically set so that the range of gear ratios available with the chainrings overlap with the range of gear ratios available with the sprockets. For example, if a front derailleur system has two chainrings (“high ratio” and “low ratio”), and a rear derailleur is used with 11 sprockets, then the lowest 6 gear ratios available with the high ratio chainring may be lower than the highest gear ratio available with the low ratio chainring. Such a bicycle can pose a number of problems for a cyclist:
Cyclists often do not know what sprocket or chainring is engaged at any particular time. This is particularly so in the dark or when riding tandem.
A cyclist may find that with the high ratio chainring none of the available gear ratios are suitable. There may be a more suitable gear ratio with the low ratio chainring but the cyclist must have information available to decide whether or not to change to the low ratio chainring.
It can be difficult for a cyclist to determine the best gear ratio if the speed of the cyclist changes rapidly, for example when slowing down for an obstruction.